1. Field of the Invention
This invention relates to the manufacture of an alkali metal electrochemical cell or battery, and more particularly to new and improved methods of preparing silver vanadium oxide composite cathode material for use in high energy density batteries.
2. Prior Art
Solid cathode, liquid organic electrolyte alkali metal or lithium anode cells or batteries are used as the power source for implantable medical devices. The cathode of such a cell may have as active material carbon fluoride, a metal oxide, a mixed metal oxide such as silver vanadium oxide, or other suitable material as disclosed in U.S. Pat. No. 4,830,940 to Keister et al., which patent is assigned to the assignee of the present invention and is incorporated herein by reference.
Preparation of cathode material containing a metal oxide such as silver vanadium oxide (AgO.sub.2 V.sub.5.5, SV0) for use in a lithium cell or battery has been known by a decomposition reaction. However, the decomposition method of preparation is accompanied by the evolution of gaseous products. Typically, the synthesis is carried out by first thermally decomposing a vanadium salt to produce vanadium pentoxide. A decomposable metal salt, preferably containing silver, is then blended with the vanadium pentoxide and the mixture is oven-dried. Following drying, the mixture is again blended and ground to ensure thorough intermingling of the constituents and the resulting homogeneous mixture is subsequently baked for a final heating/decomposition period. Depending on the starting materials used, this final heating/decomposition period can result in the release of toxic by-product gases. For example, if silver nitrate (AgNO.sub.3) is used as the silver source, nitrous oxide type gases may be formed. Such a decomposition reaction for the preparation of silver vanadium oxide is: EQU AgNO.sub.3 +V.sub.2 O.sub.5 .fwdarw.AgV.sub.2 O.sub.5.5 +NO.sub.2 +1/2O.sub.2
Upon cooling, the baked material is blended with appropriate amounts of carbon black and graphite powder to enhance conductivity, and with a binder material, and then pressed to form the cathode. Such a method is described in more detail in U.S. Pat. No. 4,310,609, to Liang et al., which patent is assigned to the assignee of the present invention and is incorporated herein by reference.
Accordingly, the present invention provides an alternate preparation method for silver vanadium oxide by chemical addition, reaction, or otherwise intimate contact of a silver-containing component with a vanadium-containing compound to form a mixed metal oxide. One such method of the present invention comprises intimately combining elemental silver Ag(0) with vanadium pentoxide V.sub.2 O.sub.5 in about a 1:1 mole ratio followed by thermal treatment of the mixed metal oxide to produce an oxygen deficient silver vanadium oxide product.
The addition reaction may be carried out at temperatures of from between about 300.degree. C. to 700.degree. C., preferably at temperatures of between about 350.degree. C. to 550.degree. C., and most preferably at temperatures of between about 380.degree. C. to 400.degree. C. and may proceed to completion within about 2 to 22 hours. Lower temperatures require a longer reaction time. Longer reaction times and higher temperatures may lead to diminished rate capability in cells utilizing the silver vanadium oxide material due to undesirable grain growth. Therefore, in general, lower temperatures are preferred in order to limit grain growth in the silver vanadium oxide product. The reaction is preferably carried out in an oxidizing atmosphere that can include air and oxygen, or the reaction may be carried out in an inert atmosphere including for example argon, nitrogen and helium. In addition, the reaction is carried out in a manner so as to provide a preselected desired shape in the electrical discharge characteristics of the electrochemical cell having a cathode containing the silver vanadium oxide material.
References related to the art of thermal treatment of silver and vanadium containing compound mixtures include:
K. Takada et al. "Eur. J. Solid State Inorg. Chem.", 28:533-545, 1991, and K. Takada et al. "(Solid State Ionics 40/41:988-992, 1990, and U.S. Pat. No. 4,965,151 to Takada et al., which describe the snythesis of silver vanadium oxides prepared by mixing powdered silver and vanadium pentoxide heated under vacuum in a sealed quartz tube. As shown in FIG. 1, heating the starting ingredients under vacuum gives a much different material than that formed by the method of the present invention (FIG. 2), as evidenced by the X-ray diffraction patterns. FIG. 1 shows X-ray powder diffraction data from silver vanadium oxide made according to Takada under vacuum and at various ratios of silver to vanadium. In comparison, FIG. 2 is a compilation of actual patterns plotted as intensity percent (I%) versus the X-ray diffraction angle (20), wherein Ar is derived from silver oxide (Ag.sub.2 O) and vanadium oxide (V.sub.2 O.sub.5) combined under argon for 24 hours at 375.degree. C. for a final stoichiometry of Ag.sub.0.7 V.sub.2 O.sub.x ; Amb is derrived from silver oxide and vanadium oxide combined under ambient atmosphere at 375.degree. for a final ratio of Ag.sub.0.7 V.sub.2 O.sub.x ; Ag is derived from silver metal and vanadium oxide combined at 600.degree. C. under argon for 48 hours for a final ratio of Ag.sub.0.7 V.sub.2 O.sub.x ; and AgNO.sub.3 is derived from silver nitrate and vanadium oxide combined in water and then baked for 16 hours at 375.degree. under ambient atmosphere for a final composition of Ag.sub.0.7 V.sub.2 O.sub.x.
Published European patent application 0 478 302 A2 discloses silver vanadium oxide cathode material for electrochemical cells and prepared by a chemical combination or addition reaction, but it does not disclose carrying out the reaction in a manner so as to provide a preselected desired shape for the electrical discharge characteristics of the electrochemical cell having a cathode containing the silver vanadium oxide.
It is, therefore, an object of this invention to provide new and improved methods for preparing a silver vanadium oxide composite cathode for use in high energy density batteries.
It is a further object of this invention to provide improved chemical control for the formation of mixed metal oxide cathode material.
It is still another object of this invention to provide new and improved methods for preparing silver vanadium oxide using elemental silver powder and V.sub.2 O.sub.5 as starting materials.